The prevalence of diabetes has become an increasing concern to the world's population. An estimated 285 million people, corresponding to 6.4% of the world's adult population, will live with diabetes in 2010. The number is expected to grow to 438 million by 2030, corresponding to 7.8% of the adult population. Diabetes is characterized by a chronic metabolic disorder that is caused by failure of the body to produce insulin and/or an inability of the body to respond adequately to circulating insulin. Secreted by the pancreas, insulin increases the ability of tissue to absorb blood glucose. Accordingly, disruption of insulin function results in the high level of blood glucose that is commonly associated with diabetic patients. There are two generally recognized form of diabetes: Type 1 diabetes, or insulin-dependent diabetes mellitus (IDDM), is characterized as an autoimmune disease involving pancreatic β-cells, while type 2 diabetes, or noninsulin-dependent diabetes mellitus (NIDDM), is characterized by β-cell dysfunction and insulin resistance. Type 2 diabetes is the most prevalent abnormality of glucose homeostasis, accounting for approximately 90-95% of all cases of diabetes. The diabetes has been widespread throughout the whole world due to ageing populations and rapid cultural changes such as increasing urbanization, dietary change, decreased physical activity and other unhealthy behavioral patterns.
The burden of diabetes is driven by vascular complications such as cardiovascular disease, stroke, nephropathy, retinopathy, renal failure, and lower limb infection and gangrene. Although these complications result from multiple metabolic disorders, hyperglycemia is considered as the main cause of both the vascular consequences of the disease and the progressive nature of diabetes itself. Most harmful of all is that high glucose levels aggravate insulin resistance, impair β-cell function and finally contribute to β-cell apoptosis. The loss of β-cell function deteriorates hyperglycemia, resulting in a vicious cycle that culminates in the abject destruction of the β-cells. The United Kingdom Prevention of Diabetes Study (UKPDS) showed that incremental reductions in glycosylated hemoglobin (HbAlC) lowered the risk of diabetes-related events [Stratton, I. M. et al. Br. Med. J. 2000, 321, 405-412]. Thus, it is recommended that patients with type 2 diabetes should reduce HbAlC values to 7% and less.
The most important strategy for treatment of type 2 diabetes involves lifestyle interventions that promote body weight loss, leading to an improvement in glycemic control. In case lifestyle interventions are not enough for the management of diabetes, an extensive range of antidiabetic drugs might be considered for the treatment of the condition (monotherapies and combination therapies). These therapies target the liver to reduce glucose output, small intestine to decrease glucose absorption, adipose deposits or muscle to elevate glucose cellular uptake or to promote glucose metabolism, serum proteases to prolong incretin action, and the pancreas to enhance insulin release. Despite the wide range of antihyperglycemic agent, it is difficult for many patients to achieve HbAlC target level. In a study reviewing diabetic patients for control of vascular risk factors, only 37.0% of participants achieved the target goal of HbAlC level of less than 7.0% [Saydah, S. H. et al. J. Am. Med. Assoc. 2004, 291, 335-342]. In addition, current therapies have limited durability and/or are associated with significant side effects such as gastrointestinal intolerance, hypoglycemia, weight gain, lactic acidosis and edema. Thus, significant unmet medical needs still remain for the treatment of diabetes. In particular, safer, better tolerated medications which provide increased efficacy and long-term durability are desired.
The obvious need for new approaches to treat patients with uncontrolled type 2 diabetes has promoted continuous exploration of alternative targets in organs involved in maintenance of glucose homeostasis. In the context of type 2 diabetes, renal glucose reabsorption contributes to plasma glucose levels and the concomitant microvascular complications. Evaluation of molecular targets available in the kidney (a major unexploited contributor to glucose homeostasis) stimulated interest in the development of a new class of antihyperglycemic agents that promote urinary glucose excretion. Inhibitors of the SGLT2 prevent renal glucose reabsorption from the glomerular filtrate and provide an insulin-independent way of controlling hyperglycemia.
Sodium-dependent glucose cotransporters (SGLTs) couple the transport of glucose against a concentration gradient with the simultaneous transport of Na+ down a concentration gradient. Two important SGLT isoforms have been cloned and identified as SGLT1 and SGLT2. SGLT1 is located in the gut, kidney, and heart where its expression regulates cardiac glucose transport. SGLT1 is a high-affinity, low-capacity transporter and therefore accounts for only a small fraction of renal glucose reabsorption. In contrast, SGLT2 is a low-affinity, high-capacity transporter located exclusively at the apical domain of the epithelial cells in the early proximal convoluted tubule. In healthy individuals, greater than 99% of the plasma glucose that filtered in the kidney glomerulus is reabsorbed, resulting in less than 1% of the total filtered glucose being excreted in urine. It is estimated that 90% of renal glucose reabsorption is facilitated by SGLT2; the remaining 10% is likely mediated by SGLT1 in the late proximal straight tubule. Genetic mutations in SGLT2 lead to increased renal glucose excretion of as much as 140 g/day depending on the mutation with no apparent adverse effects on carbohydrate metabolism. Since SGLT2 appears to be responsible for the majority of renal glucose reabsorption based on human mutation studies, it has become a target of therapeutic interest [Lee, J. et al. Bioorg. Med. Chem. 2010, 18, 2178-2194; van den Heuvel, L. P. et al. Hum. Genet. 2020, 111, 544-547].
Phlorizin was isolated from the root bark of the apple tree and evaluated as the first SGLT inhibitor. Despite antidiabetic potency of phlorizin, its metabolic instability due to β-glucosidase cleavage in the intestinal tract has prevented its development as a drug for the treatment of diabetes. Subsequently, T-1095, by Tanabe Seiyaku, was reported as the first orally absorbable SGLT2 inhibitor, overcoming the disadvantage of phlorizin. T-1095 was absorbed in the intestine and converted to an active form, T-1095A. Following the discovery of T-1095, O-aryl glucosides such as sergliflozin and remogliflozin advanced furthest in clinical trials. Again, concern regarding gut β-glucosidase-mediated degradation, resulted in developing sergliflozin A and remogliflozin A being administered as the ethyl carbonate prodrugs sergliflozin and remogliflozin, respectively. Subsequent endeavors to identify SGLT2 inhibitors suitable for oral administration without the need for a prodrug led to the discovery of C-aryl glucoside-derived SGLT2 inhibitors. C-aryl glucoside appears to have drug-like properties with enhanced chemical stability of the glucosidic bond. Extensive SAR studies by Bristol-Myers Squibb identified dapagliflozin, a potent, selective SGLT2 inhibitor for the treatment of type 2 diabetes. At present, dapagliflozin is the most advanced SGLT2 inhibitor in clinical trials and is believed to be the first SGLT2 inhibitor to go to market [Meng, W. et al. J. Med. Chem. 2008, 51, 1145-1149]. On the other hand, Mitsubishi Tanabe Pharma, in collaboration with Johnson & Johnson, is developing canagliflozin, another novel C-aryl glucoside-derived SGLT2 inhibitor [Tanabe Seiyaku, WO2008013321].
Considering the important impact of diabetes on public health and unmet medical needs of current therapy, it is no surprise that SGLT2 inhibitors are currently interesting topics of studies, which were published in the following review articles [Washburn, W. N. Expert Opin. Ther. Patents, 2009, 19, 1485-1499; Washburn, W. N. J. Med. Chem. 2009, 52, 1785-1794].